The present invention relates to non-linear optics and, more particularly, to an apparatus for frequency conversion of light based on a diode laser structure.
Semiconductor lasers play an important role in optical fiber transmission systems, signal amplification systems, wavelength division multiplexing transmission systems, wavelength division switching systems, wavelength cross-connection systems and the like. In addition semiconductor lasers are useful in the field of optical measurements.
A semiconductor laser (first proposed in 1959) is based on current injection of non-equilibrium carriers into a semiconductor active medium, resulting in population inversion and sufficient modal gain to achieve lasing.
Referring now to the drawings, there are basically two types of semiconductor lasers which presently dominate the laser market, which are depicted in FIGS. 1a-b. FIG. 1a depicts a Vertical Cavity Surface-Emitting Laser (VCSEL), where the photons arc cycled in a high finesse cavity in vertical direction (upward in FIG. 1a). In this laser, the cavity is very short and the gain per cycle is very low. Thus, it is of key importance to ensure very low losses at each reflection, otherwise, lasing will either not be possible, or will require too large current densities, not suitable for continuous wave operation. Since first proposed in 1962, VCSELs have become very popular. VCSELs can be made small, may operate at low threshold currents and are produced in a very production-friendly planar technology.
Another type of semiconductor laser is an edge-emitting laser, which is depicted in FIG. 1b. In this laser, an active medium (e.g., a thin layer) is placed in a waveguide having a larger refractive index than the surrounding cladding layers, to ensure a confinement of the laser light in the waveguide. The produced light is diffracted at the facet exit of the device at typically large angles of 30°-60°. The advantage of the edge-emitting laser is its compact output aperture which is realized simultaneously with high light output power. The disadvantage of the edge-emitting laser over the VCSEL is astigmatism phenomenon often occurring when circular output aperture are employed. Additionally, as opposed to the VCSEL, in the edge-emitting laser a temperature increase results in a significant wavelength shift caused by the bandgap narrowing of semiconductors with increasing temperature.
One of shortcomings of all semiconductor lasers is that the wavelengths (or the frequencies) of the emitted light are limited to values provided by the values of the energy bandgap of semiconductor materials. The available wavelengths may additionally be shifted to larger values (the so called red shift), due to localization of carriers by various structures known as quantum well-, quantum wire- or quantum dot heterostructures. The semiconductor laser technology is well developed for III-V compound semiconductors and cover wavelengths beyond 600 nm. Presently known semiconductor lasers below 600 nm (e.g., in the ultraviolet to green spectral range) are much less mature.
An additional disadvantage of the semiconductors lasers is a poor beam quality, wide spectrum and poor temperature stability of the wavelength.
Several methods have been proposed to generate light below 600 nm, basically using non-linear optical techniques which convert the wavelength of the light outputted from the semiconductor laser. These techniques are capable of generation light in an extremely broad spectral range, e.g., from mid-infrared (mid-IR) to visible light. Examples of frequency conversion techniques include sum frequency generation (SFG), frequency doubling (which is a special case of SFG), differential-frequency generation (DFG) and optical parametric generation.
Over the past decade, processes of frequency conversion have become commercial available with the manufacturing of products such as frequency-doubled green sources replacing multi-Watt Ar+ ion lasers and optical parametric oscillators generating mid-IR radiation at enhanced power level for defense applications.
For example, U.S. Pat. No. 5,175,741, the contents of which are hereby incorporated by reference discloses a wavelength conversion method employing a single nonlinear optical (NLO) crystal. A solid-state laser pumped by a semiconductor laser and produces a laser beam which is oscillated by the solid-state laser. The NLO crystal then converts the wavelengths of a laser beam and the wavelength of a pumping laser beam into the wavelength of a wave whose frequency is the sum of the frequencies of the laser beams.
The need of solid-state lasers in frequency conversion processes is generally motivated by several arguments. First, a solid-state laser provides a high quality laser beam with fairly low beam divergence and low astigmatism. Second, the spectral width of the laser beam is sufficiently small to allow a maximum wavelength conversion efficiency of the NLO crystal. For example, for a KNbO3 crystal, the full width at half maximum of the conversion efficiency peak is typically about 0.5 nm. Thus, solid-state lasers with spectral widths below 0.1 nm are well suited for frequency conversion by KNbO3.
However, the above techniques suffer from the following inefficiency limitation. The maximum power conversion efficiency for the light conversion from a semiconductor diode laser to a solid-state laser is not higher than 30%. On the other hand, the frequency conversion efficiency of the solid state laser to the second harmonic using an NLO crystal can be as high as 70%. Thus, the inefficiency of the process is originated in the step of converting the diode laser (or lamp) light to the solid-state laser light.
Proposed techniques for improving efficiency are disclosed, e.g., in U.S. Pat. Nos. 5,991,317 and 6,241,720, the contents of which are hereby incorporated by reference. In these techniques, the concept of an intra-cavity conversion is employed. For example, U.S. Pat. No. 5,991,317 discloses a resonator cavity defined by two or more resonator mirrors. A laser crystal and several NLO crystals are positioned in the resonator cavity. A diode pump source supplies a pump beam to a laser crystal and generates a laser beam with a plurality of axial modes impinging the NLO crystals and producing a frequency doubled (or tripled) output beam.
However, the conversion efficiency of these techniques is still rather low. It is recognized that the low conversion efficiency requires the use of high power diode lasers, which inevitably have to be cooled. Thus, the inefficiency problem is aggravated by the energy loss due to heating which is at least 90% of the total energy.
In addition, the optimal wavelength of the NLO crystal for the conversion efficiency depends on the temperature (for example, for KNbO3 the optimal wavelength is 0.28 nm/° K). This is in contradiction to the solid-state laser in which the wavelength is stable. For an efficient operation, the temperature of the NLO crystal is to be precisely controlled by adding components to the system thereby increasing the complexity of the design.
Another disadvantage is the fact that the solid-state lasers have a strictly defined wavelength, limiting the possibility to get an arbitrary frequency converted wavelength.
In the above technique, the diode laser is used for pumping while the frequency conversion is performed indirectly using the solid-state laser. An alternative solution for improving the efficiency of frequency conversion is to use edge-emitting diode lasers for a direct frequency conversion. However, for such lasers the matching between the laser wavelength and the optimal NLO crystal wavelength is extremely difficult, first because of the broad spectrum of the produced light and second because the lasing wavelength is temperature-dependent.
Another disadvantage is the very high beam divergence of a diode laser. This divergence causes strong deviation of the laser beam with respect to the required crystallographic direction and additionally ruins the performance of the device.
Correction of the beam divergence typically requires a complicated setup involving a few lenses, which are so positioned to focus the pump radiation onto the surface of the NLO crystal [to this end see, e.g., Simon, U. et al., “Difference-Frequency Generation in AgGaS2 by Use of Single-Mode Diode-Laser Pump Sources”, Optics Letters, 18, No. 13:1062-1064, 1993 and U.S. Pat. Nos. 5,912,910, 6,229,828, and 6,304,585]. However, the additional lenses, which are used for converting the laser output to a parallel beam, are known to cause a significant broadening of the beam diameter hence to reduce the power density, which is a key requirement for efficient wavelength conversion. As a result of these problems, edge-emitting diode lasers are not used commercially for direct frequency conversion and applied mostly as pumping sources for solid state lasers.
Still another system employing semiconductor diode lasers for a direct frequency conversion is disclosed in U.S. Pat. No. 6,097,540. In this system, beams generated by many lasers are combined to a single beam by a system of lenses and mirrors and directed onto a surface of a NLO crystal. However, this solution does not provide a significant advantage over the above techniques, as the proposed system is very complex and expensive, contains a large number of lasers, provides only an extra-cavity conversion and is not wavelength stabilized.
There is thus a widely recognized need for, and it would be highly advantageous to have, an apparatus for frequency conversion devoid of the above limitations.